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Downloaded 10/06/21 01:33 PM UTC 1070 MONTHLY WEATHER REVIEW VOLUME 144 MARCH 2016 M A S H I K O 1069 A Numerical Study of the 6 May 2012 Tsukuba City Supercell Tornado. Part I: Vorticity Sources of Low-Level and Midlevel Mesocyclones WATARU MASHIKO Meteorological Research Institute, Tsukuba, Japan (Manuscript received 29 March 2015, in final form 6 December 2015) ABSTRACT On 6 May 2012, an F3 supercell tornado, one of the most destructive tornadoes ever recorded in Japan, hit Tsukuba City in eastern Japan and caused severe damage. To clarify the generation mechanisms of the tornadic storm and tornado, high-resolution numerical simulations were conducted under realistic environ- mental conditions using triply nested grids. The innermost simulation with a 50-m mesh successfully repro- duced the Tsukuba City tornadic supercell storm. In this study (the first of a two-part study), the vorticity sources responsible for mesocyclogenesis prior to tornadogenesis were investigated by analyzing vortex lines and the evolution of circulation of the mesocy- clones. Vortex lines that passed through the midlevel mesocyclone (4-km height) originated from the envi- ronmental streamwise vorticity, whereas the low-level mesocyclone and low-level mesoanticyclone were connected by several arching vortex lines over the rear-flank downdraft associated with the hook-shaped distribution of hydrometeors (hereafter hook echo). Most of the circulation for the circuit surrounding the midlevel mesocyclone was conserved, although the baroclinity associated with positive buoyancy within the storm led to an up-and-down trend. The circulation of the material circuit encircling the low-level mesocy- clone showed a gradual increase caused by baroclinity along the forward-flank gust front. Friction also had a positive net effect on the circulation. In contrast, most of the negative circulation of the low-level meso- anticyclone was rapidly acquired owing to baroclinity around the tip of the hook echo. Just after tornado- genesis, the low-level mesocyclone intensified significantly and developed upward, which caused retrograde motion of the midlevel mesocyclone. 1. Introduction sometimes observed on the anticyclonic shear side of the rear-flank downdraft (RFD) outflow as a counterpart Our understanding of the structure, evolution, and of the low-level mesocyclone (e.g., Brandes 1981; dynamics of supercell storms has been greatly advanced Markowski et al. 2008, 2012a; Atkins et al. 2012). Sev- by observational, numerical, and theoretical studies eral observational and numerical studies have shown conducted over the past few decades. As many previous that supercell tornadogenesis is preceded by the in- studies (e.g., Browning 1964; Lemon and Doswell 1979; tensification of the low-level mesocyclone (Rasmussen Klemp 1987) have indicated, supercells are character- et al. 2000; Mashiko et al. 2009; Schumacher and ized by the existence of a persistent mesocyclone with a Boustead 2011; Schenkman et al. 2014) because the strong updraft within a convective storm. In the early dynamically induced pressure deficit associated with the stage of supercell storms, a strong rotating updraft forms low-level mesocyclone intensifies the updraft near at midlevel (;4-km height, hereafter referred to as a the surface (e.g., Wicker and Wilhelmson 1995; Noda midlevel mesocyclone). As the storm develops, a low- and Niino 2010). Although numerous observational level mesocyclone (;1-km height) becomes prominent, studies (e.g., Wakimoto and Cai 2000; Markowski et al. and a low-level mesoanticyclone (;1-km height) is 2002, 2011; Wakimoto et al. 2004) have reported that nontornadic supercell storms are often similar in struc- ture and evolution to tornadic supercells, Trapp et al. Corresponding author address: Wataru Mashiko, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, (2005) showed statistically that more than 40% of low- Japan. level mesocyclones detected by a Doppler radar net- E-mail: [email protected] work in the United States are associated with tornadoes. DOI: 10.1175/MWR-D-15-0123.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/06/21 01:33 PM UTC 1070 MONTHLY WEATHER REVIEW VOLUME 144 Thus, an understanding of the formation mechanisms of vorticity. Numerical studies of supercell storms con- mesocyclones not only increases scientific knowledge ducted during the 1990s or earlier (e.g., Klemp and but also is crucial for improving operational tornado Rotunno 1983; Wicker and Wilhelmson 1995) also sug- warning systems. Nevertheless, the formation mecha- gested that the baroclinity along the forward-flank gust nisms of low-level mesocyclones remain unclear, and front (FFGF) might be a dominant vorticity source re- one of the most fundamental uncertainties pertains to sponsible for the development of low-level mesocy- the vorticity sources responsible for mesocyclogenesis in clones. Wicker (1996) indicated that the interaction supercell storms at low levels. between the environmental horizontal vorticity near the The vorticity source of midlevel mesocyclones in surface and the baroclinically generated horizontal supercell storms is relatively well understood. Using vorticity along gust fronts is crucial for the development vortex line analyses, numerous previous studies (Davies- of a low-level mesocyclone. Rotunno and Klemp (1985) Jones 1984; Rotunno and Klemp 1985; Markowski et al. analyzed the circulation of a material circuit surrounding a 2008, 2012a) revealed that midlevel mesocyclones ac- low-level vortex and estimated the baroclinic contribu- quire vertical vorticity aloft by the tilting of horizontal tion directly. They found that the circulation originated vorticity associated with environmental vertical wind mostly from baroclinity along the gust fronts. However, shear. Indeed, storm-relative environmental helicity the finding of strong baroclinity along gust fronts (SREH) (e.g., Davies-Jones et al. 1990), which is calcu- within a storm in the aforementioned simulation results lated by integrating vertically the scalar product of the is disputable, because the cold pools simulated behind environmental horizontal vorticity and storm-relative the gust fronts were excessively strong compared to wind vectors, is frequently used as an index of the po- those reported by observation (e.g., Davies-Jones 2006; tential for supercell genesis. Although the possible im- Shabbott and Markowski 2006). portance of the baroclinic effect on a midlevel rotation More recent idealized numerical studies have in- was also acknowledged (Davies-Jones et al. 2001), the dicated that the RFD and/or the FFD plays a dominant contribution of the environmental vertical wind shear to role in creating vertical vorticity and bringing it to the the vorticity source of a midlevel mesocyclone has thus ground (Dahl et al. 2014; Markowski and Richardson far not been quantified by analyzing the circulation of the 2014; Parker and Dahl 2015). The baroclinically gener- material circuit tracked backward from the midlevel ated horizontal vorticity is tilted upward during the mesocyclone. parcel descent. Markowski and Richardson (2014) also Numerous studies of low-level mesocyclones have analyzed the vorticity forcing along a trajectory initiated focused on the RFD as the vorticity source. Vortex lines from a region of negative vertical vorticity associated passing through low-level mesocyclones form arches with a near-surface anticyclonic vortex. The vorticity over the RFD region, which suggests that the vorticity is vector generated by baroclinity is inclined below the baroclinically generated by horizontal buoyancy gradi- descending parcel trajectory. These results are consis- ents in the RFD region (Straka et al. 2007; Markowski tent with previous studies showing arching vortex lines et al. 2008, 2012a). If the leading edge of the vortex rings over the RFD region (Straka et al. 2007; Markowski produced by baroclinity associated with the RFD region et al. 2008, 2012a). is lifted by updrafts due to the gust front or the low-level However, these idealized numerical studies of the mesocyclone, arching vortex lines form and connect the low-level mesocyclones and mesoanticyclones did not cyclonic and anticyclonic vortices. These storm- evaluate the frictional effect despite their consideration generated vortex lines and the environmental vorticity of near-surface phenomena; the circulation of material usually have different orientations (e.g., Markowski circuits was changed solely by baroclinity and turbulent et al. 2008). mixing while neglecting surface friction. Using Doppler radar data, Markowski et al. (2012b) In this study, which is the first part of a two-part quantitatively evaluated the vorticity sources of a low- study, high-resolution simulation results were used to level mesocyclone in a supercell by analyzing the cir- investigate a tornadic supercell that caused severe culation of the material circuit surrounding the vertical damage to Tsukuba City, Japan, on 6 May 2012. A vorticity maximum associated with the low-level meso- simulation with a 50-m horizontal grid spacing was cyclone. They revealed that the baroclinity associated conducted under realistic environmental conditions that with the forward-flank downdraft (FFD) in the pre- included the surface drag. The aim of the study is to cipitation region is the primary source of circulation, and quantify the vorticity sources of the low-level and mid- that the RFD associated with the descending reflectivity level mesocyclones and the low-level mesoanticyclone core (e.g., Byko et al. 2009)
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